The present invention relates to in-vehicle navigation systems and, more particularly, to in-vehicle navigation systems employing a terrestrial magnetic sensor (magnetic compass) aboard the vehicle to derive at least part of its guidance information.
Because a motor vehicle contains large amounts of iron and steel, a magnetic compass on board the vehicle suffers the effects of a local magnetic field and cannot point even to magnetic north, let alone to true north. Some form of calibration of the compass is required before it can be used reliably to show direction.
Japanese Patent Application First Publication No. (Tokkai) Sho 58-48811 proposes a method for calibrating a magnetic compass on board a vehicle. However, this method, like similar methods long used aboard ships, requires that the vehicle travel in a complete circle. Further, the calibration that results is valid only in the immediate vicinity of the place where it was performed, because the local magnetic field of the vehicle is both time- and location-dependent. Compensation for long-term use cannot rely on a single calibration.
In a hybrid vehicle navigation system that contains both a magnetic compass and a Global Positioning System ("GPS") receiver, however, true heading can be determined from GPS signals and used to calibrate the compass in real time.
U.S. Pat. No. 4,743,913 discloses several methods for compensating magnetic compass headings from a GPS-derived absolute heading. These methods require that the compass be of the flux-gate type.
In the first method, one determines the center of the magnetic circle from the compass coil voltages of a flux-gate compass by passing two lines through the coil voltages and determining their slope from the absolute headings derived from signals from the GPS.
The second method avoids incorrect headings by measuring absolute heading only when the vehicle travels along a straight road, where straightness is determined by a steering-wheel sensor or other signal processing means.
In the third method, one assumes that the radius of the circle of magnetization, which radius corresponds to the strength of the magnetic field; the compass coil voltages; and the absolute heading for a single point (derived from the GPS signals), are known. The center of the magnetization circle then lies along a line, whose slope is determined from the absolute heading, at a distance from the single point that is equal to the radius of the circle.
All these methods require compass coil voltages from, for example, a flux gate compass. Also, it is necessary to know the strength of the magnetic field in order to use the above methods. In some systems, it is more convenient to process separately the X and Y outputs of the flux gate compass to produce a magnetic heading signal. This is not a satisfactory input for the referenced system. Hence the methods presently known have significant disadvantages.